Mobile communications may be in continuous evolution, for example we are at the doorstep of its fifth incarnation (e.g., 5G). As with previous generations, new use cases and design goals may drive many new, often competing requirements for the system. Building upon basic technologies already known from conventional Orthogonal Frequency-Division Multiple Access (OFDMA) and legacy Long Term Evolution (LTE) systems, the principles and operation of a flexible radio access system for 5G are described.